1. Field of the Invention
The present invention relates to a developing unit and an image forming device.
2. Description of the Related Art
A known image forming device used for a copier or a printer includes a developing unit for developing an electrostatic latent image formed on a photoreceptor with toner supplied to the photoreceptor. In such a device, the toner is transferred to the photoreceptor from a toner layer formed on a surface of a developing roller. The developing roller is a cylindrical electric conductor provided in the developing unit.
In such an image forming device, voltage is applied to the device to produce a predetermined potential difference between the developer roller and the photoreceptor in order to transfer the toner from the developing roller to the photoreceptor.
However, if the developing roller and the photoreceptor are disposed close to each other with the voltage applied to produce a predetermined potential difference therebetween, leakage of electric current from between the developing roller and the photoreceptor may occur, so that it is difficult to form a toner image which accurately represents the electrostatic latent image on the photoreceptor, and a toner layer on the surface of the developing roller.
Japanese Unexamined Patent Application Publication No. 2004-318092 discloses a method for preventing leakage of electric current from between the developing roller and the photoreceptor by covering a cylindrical surface of the developing roller with an alumite layer (i.e., an aluminum oxide layer).
Alumite, however, has a high electric-resistivity, which may cause excessive charging of the toner due to friction between the toner and the developing roller. This tendency is particularly evident in positively charged toner. As a result, the toner turnover efficiency may become lower.
To address the problem described above, a layer (i.e., a low-resistance layer) made of a material with an electric-resistivity lower than that of alumite may be formed on the alumite layer so as to prevent charge injection into the toner and thus prevent excessive charging of the toner.
If a low-resistance layer is formed on top of the alumite layer, however, electric current may leak into the low-resistance layer from a portion of the electric conductor which forms a core material at an end of the developing roller. The electric current may also leak from the low-resistance layer into the photoreceptor.
The present invention has been made in view of the aforementioned circumstances. Accordingly, it is an object of the present invention to prevent defective toner transfer to the photoreceptor due to excessive charging of the toner. It is another object of the present invention to prevent leakage of electric current between a developing roller and a photoreceptor.